Family: Ascoviridae

Genus: Ascovirus

Distinguishing features

Ascoviruses typically replicate in lepidopteran larvae of the family Noctuidae producing virion-containing vesicles, which give a milky white colouration to the haemolymph. Virions of ascoviruses are either bacilliform, ovoidal or allantoid in shape measuring 130 nm in diameter by 200–400 nm in length. 

Virion

See discussion under family description. 

Genome organization and replication 

See discussion under family description. 

Biology

See discussion under family description. 

Species demarcation criteria

The following list of characters is used in combination to differentiate species in the genus:

• Phylogenetic position of genes encoding homologs of IIV6 ORFs 022L, 037L, 067R, 075L, 142R, 176R, 295L, 347L, 393L and 428L
• Presence or absence of occlusion bodies
• Lack of DNA/DNA hybridization with other species at low stringency
• Restriction enzyme fragment length polymorphisms (RFLPs)
• Host of isolation and experimental host range
• Tissue tropism
• Association with specific hymenopteran parasites, if apparent

Members of the family Ascoviridae, in particular members of the genus Ascovirus, can have broad host ranges among the larvae of lepidopteran species, and the fat body tissue is a major site of replication for most species (Federici and Govindarajan 1990). The above characters are therefore used in combination to distinguish existing and new ascovirus species from one another. Hybridization studies have proven particularly useful, and when combined with RFLPs and phylogenetics can also be used to distinguish variants within a species. For example, SfAV-1a DNA does not hybridize with HvAV-3a or TnAV-2a DNAs under conditions of low stringency. TnAV-2a DNA hybridizes to some extent with HvAV-3a DNA, but not as strongly as it does with homologous DNA. In addition, TnAV-2a replicates in a range of larval tissues including the fat body, tracheal matrix and epidermis, but SfAV-1a and HvAV-3a appear to replicate, respectively, only or primarily in the fat body tissue of most hosts. SfAV-1a virions are bacilliform and are occluded in vesiculate occlusion bodies, whereas TnAV-2a virions are allantoid and are not occluded in occlusion bodies. HvAV-3a virions vary from allantoid to bacilliform, and are not occluded in occlusion bodies. 

When the genome of a new isolate cross-hybridizes with that of an existing species member, RFLPs can be used to distinguish variants. Numerous ascovirus isolates, for example, have been obtained from larvae of different noctuid species, including Heliothis virescens, Helicoverpa zea, Autographa precationis and Spodoptera exigua in the United States, as well as from Helicoverpa and Spodoptera species in Australia, China and Indonesia. The DNA of many of these isolates shows strong reciprocal hybridization with HvAV-3a DNA under conditions of high stringency. RFLP profiles of these isolates, however, often show variations from HvAV-3a that range from minor to major. Because these isolates cross-hybridize strongly with HvAV-3a, they are considered variants of this viral species. Moreover, experimentally these isolates have been shown to have host ranges that overlap with HvAV-3a, providing additional evidence that they are variants of the same species. A similar situation occurs with isolates of TnAV-2a and TnAV-2b. 

Related, unclassified viruses

Virus names and virus abbreviations are not official ICTV designations.